Machining Rocket Nozzles


    Rocket Nozzles are one of the most diverse parts of making rocket engines, they can be either as simple as a hole drilled into clay, or a Graphite machined nozzle with a liquid cooling system. I usually use Bentonite clay nozzles for simple motors, but when im working with high powered rocket engines, I usually tend to make my nozzles out of Graphite with a 45° compression (convergent) cone, and 15° expansion (divergent) cone; no liquid cooling (although that might be a neat future project ;-).  One of the great properties of Graphite nozzles is that Graphite can withstand extreme temperatures, and is not very reactive, so even the acidic byproducts of an ammonium perchlorate composite propellant will not deteriorate it; although they still do need replaced after 12 to 20 firings. Other Nozzles inc

    To begin making the Graphite nozzles we first need a metal lathe, mine is a Cummins Mini-lathe, I know, its not the best, but all I can afford at the time, still being in high school and all. Currently I am looking for a new lathe, thinking of an Atlas or South Bend (would get a Hardinge, but I can afford a $10,000 lathe right now).
Cummins Mini Lathe 7 X 12     1" 6061-T6 Aluminum Alloy Round Stock
3/4" Graphite Round Stock (Graphite Rod)

Facing the Aluminum

Above is the aluminum being faced. This just makes it perfectly square and gives a nice working surface.

Drilling the Aluminum Stock

Now that the hole is drilled through the aluminum, it is time to hollow it out to accept the graphite, this process is just called boring.

Boring the Aluminum

Yes, as you can see I do grind my own lathe bits. I find it much cheaper to grind my own at $1 a piece, than buying pre-ground ones for $5, and they seem to cut much better. I recommend buying the lathe blanks from Enco or MSC, they have some good prices for 5/16" square HSS blanks, and as long as you have a decent bench grinder, they are a breeze to make. Enough about lathe bits, lets get to machining again. So now the aluminum has a 1" OD and a .75" ID, so lets stick in the graphite.

Graphite being inserted into the aluminum

Above is the graphite, waiting to be malloted into the aluminum. To make sure the graphite doesn't ever slip out from all the pressure, this has to be a tight fit. Also, I add some high-temp glue to aid in inserting the graphite and to make sure it is secure. Now I let the glue cure before continuing work on the piece.

Graphite flush with aluminum

Now that the glue was cured, I just faced the piece to make the graphite flush with the aluminum, and the drilled a hole in the center (not shown). Now I will machine in the 45° convergent cone and the 15° divergent cone.

Convergent cone being machined

Above is the convergent cone being drilled in with a 90° countersink. I bought this countersink off of eBay for something like $3, and it makes the process much faster and easier, so it is quite a worth investment. as for my 15° piece, its not so majestic, as you will see below.

Divergent cone being made

Yep, its just an old 7/8" auger bit that was cut to have a 15° angle on both sides. Basically it is a ghetto rigged 30° countersink. It does a nice job though.

Finally the nozzle is done, and here is the final look at it.

45 degree convergent cone view.             15 degree divergent cone view.

And now here is a view of the nozzle it its new engine.

Nozzle in engine